The present invention relates to a continuous web which by transverse cutting at selected positions and possibly by effecting an edge insulation and attaching electrical connections provides electrical low temperature heating mats, said web consisting of resistance foil material between two insulating foil layers which are joined in the areas of direct contact to secure the resistance foil, a plurality of parallel, conductive, longitudinal foil strips and transverse strips extending transversely to said longitudinal strips at desired intervals lying between the insulating layers, said transverse strips being connected to the longitudinal strips and being so constructed as to form electrical connections between adjacent longitudinal strips. The invention also relates to a method for the production of such webs.
Heating mats of the abovementioned type has hitherto been produced as disclosed in Norwegian Patent Specification 109 918 (corresponds to U.S. Pat. No. 3,263,307), i.e. by cutting a continuous web consisting of a resistance foil lying between two broader, insulating layers that are joined in the areas of direct contact, i.e. outside the resistance foil and through slots therein. The desired configuration of the resistance foil and through slots therein. The desired configuration of the resistance foil has been produced by interrupted slotting of the foil, material being removed to form longitudinal slots of uniform width, said slots being periodically interrupted over short distances. By staggering the interruptions of every second slot from the interruptions of the remaining slots a subsequent cutting of the continuous web in the areas of interruption of the slots will produce heating mats with a meander patterned resistance body have meander arms extending the full length of the mat. After the cutting, the resistance foil which extends right to the terminal edges of the mat, must be insulated along these edges, and this can e.g. be effected by applying a bead of thermoplastic material or an adhesive which solidifies upon cooling. Further, an electrical connection must be effected, i.e. the ends of the meander patterned resistance foil lying inside the mat must be connected to electrical conductors. This connection and the structure of the mat in other respects shall be such that the mat can be connected to an ordinary mains voltage of for instance 220 volts.
However, the method used in this connection for slotting a resistance web in the desired meander pattern entails some disadvantages. Thus, the fact that the slotting must be interrupted at certain intervals makes the method somewhat complicated and prone to disturbances. Further, the material removed in the forming of slots of uniform width represents an economic loss. Certainly, the material may be returned to the foil manufacturer, but the price paid for this waste material is, of course, not as high as for the finished foil. In connection with the provision of an electrical conductive connection from the resistance foil lying between the insulating layers to electrical conductors for connection to the mains, the known method of producing the heating mats also comprises some manual work which is difficult to adapt for automation.
From Swiss Patent No. 404,821 (FIG. 13) there is also known a suggestion for a heating band wherein several meander shaped current paths are provided between two insulating foils by longitudinal resistance strips of limited length and short transverse resistance strips. The band may be severed into band shaped heating elements having one or more meander patterned current paths which may be connected in parallel or in series. However, to place interrupted strips, especially short transverse strips, in a correct position will offer problems, and the patent specification does not teach how this may be done in continuous, automatic production. The patent specification also discloses (FIG. 14) another embodiment of the heating band having only longitudinal, continuous resistance strips. In this case, the transverse connections are to be established in a separate operation after the band has been severed into desired lengths. In preparation thereof, the longitudinal strips are doubled in certain intervals, e.g. by being folded back upon itself in Z folds. In order to avoid having to separate the insulating sheets when establishing the transverse connections, these connections must be formed outside the severed ends.